1. Field of the Invention
This invention relates to a projection lens system for use in a projection type display and for displaying on a screen an image formed on a face plate of a cathode ray tube (hereinafter referred to as CRT).
2. Description of the Related Arts
Recent years have seen the increase in the use of a projection type display capable of projecting a TV image in a large screen size. For such a projection display, although several systems have been adopted, a system for projecting an image, which has been formed on a CRT on a screen, through a series of projection lenses is most widely utilized among others.
In respect of the projection lenses, there has been a demand for a lens having a wider field angle in order to reduce the size of the display cabinet as small as possible.
As another demand, the projection lens is expected to have a smaller F-number to secure a sufficient brightness of the image on a screen.
Projection lenses disclosed in U.S. Pat. Nos. 4,300,817 and 4,776,681 can be enumerated as the conventional types of projection lens. The type disclosed in U.S. Pat. No. 4,300,817 comprises three lens groups, while the type disclosed in U.S. Pat. No. 4,776,681 consists of five lens groups.
Further, as the typical projection lens, there have been a projection lens composed of ten lenses or more and a projection lens unit, a partial lens group thereof is formed in aspherical shape and the rest thereof are made up of plastic.
Thus, various types of conventional projection lenses have widely been used.
However, in the projection type display apparatus for displaying an image in large screen size or a high definition TV, a development of which is now proceeding, the above-described projection lens cannot always provide an image meeting the aforementioned requirements.
With the conventional projection lens, for example, a picture distortion occurs at the corner of the image projected on the screen and the volume of the projector itself disadvantageously becomes large because of the small field angle of the lens.
Accordingly, there has been expected a projection lens which is capable of obtaining an appropriate MTF (modulation transfer function) from the center to the corner of the image on the screen as well as one which is free from the aforementioned drawbacks.
The present invention aims to solve the above-mentioned problems with the conventional devices. One object of the present invention is to provide a projection lens capable of properly correcting aberrations from the center to the periphery of the image having a sufficient F number and a satisfiable field angle.